A mermaid is a mythical marine creature that has the appearance of a (naked) woman down to the waist and a fish's tail below the waist. The word comes from mer (sea) + maid. The male counterpart is a merman.
The idea probably arose from sailors seeing marine creatures and fantasising that they were women, or at least partly women.
In 2017, a trend grew in the United Kingdom of wearing a monofin – a mermaid tail – to swim in pools and seas. It was condemned as dangerous.